This invention pertains to a method and apparatus for supporting a plasma loop, and specifically for such a method and apparatus in which voltages are applied to and plasma gas is directed about at least three electrodes which are distributed to maintain the plasma.
The purpose of the present invention, although not limited to this particular application, is to provide a hot plasma (ionized gas) region that will atomize analytical samples and excite the resulting analyte atoms so that they emit light of characteristic wavelengths. The invention can be used and practiced with a spectrometer that measures intensity of the emitted light, which measurements can then be used to determine the concentrations as chemical elements in the sample. Alternately, the atoms produced by the invention can be observed by atomic absorption or atomic fluorescence spectrometers, which can also help determine the concentrations of chemical elements in the sample.
The need for simultaneously determining high and low level concentrations of many chemical elements in a variety of different types of samples (e.g., biological, environmental, geological, industrial) has lead to the development over the last decade of emisson spectrometers capable of determining twenty to forty elements at once in a given sample. In one type of instrument known as an induction coupled plasma source, the sample in liquid form is nebulized into an argon (or other plasma gas) stream and swept into an argon plasma. Although other plasma gases can be used, argon is desirable because it is inert. The plasma is maintained by inductively coupling several kilowatts of power into the plasma from a radio-frequency power source. The high temperature of the plasma (5,000.degree.-10,000.degree. C.) atomizes the sample and excites the free atoms. Spontaneous emission from the analyte atoms is detected photoelectrically by a multi-channel (20-40 element) direct-reader spectrometer. The intensity measurements are related to concentrations of the elements in the original sample by the use of standard samples. The plasma is analogous to analytical flames used in common atomic absorption instruments. However, the much hotter temperature of the plasma reduces interferences caused by matrix effects, and increases the emissions of the analyte atoms to the point that the emission signal gives better detection limits than the atomic absorption signal.
In another design, the argon plasma is maintained by passing a direct current of five to ten ampers through the plasma between a pair of electrodes. This type of plasma is easier to generate and does not blow out as easily as the induction coupled plasma under varied operation conditions. An argon stream containing the nebulized samples is directed at the plasma. However, the hot plasma tends to repel the cooler argon stream, and consequently most of the sample passes around and does not come in contact with the hottest part of the plasma. This problem occurred initially for the induction coupled plasma, and was solved by controlling the experimental conditions so that the hot core of the plasma formed a doughnut-shaped region. The sample stream passes through the center of the doughnut forcing it to come into contact with the hotter regions of the plasma. Both the argon DC arc plasma and the induction coupled plasma are commercially available as complements of instruments costing several tens of thousands of dollars.
A third plasma-generating source of which applicant is aware is used for producing high power plasma flows. It includes a set of three electrodes having concurrent axis pointed in a direction corresponding with a desired direction of plasma flow. A pilot plasma jet is directed in the desired direction of plasma flow. Three-phase electric power is applied to the electrodes and a separate jet of plasma gas is directed longitudinally along each electrode into the main plasma jet such that a tripod-shaped plasma is generated. This plasma source, if used for sample analysis, would have the same disadvantage with respect to heating a stream of sample material as does the DC source previously described.
It is therefore a general object of the present invention to overcome many of the problems exhibited in the prior art.
In particular, it is an object to provide a plasma source which produces plasma in the form of a loop through which sample material may be directed for analysis.
It is a further object of this invention to provide such a source which may be designed to support a variety of plasma loop shapes and sizes.
It is also an object to provide a plasma source in which the thickness of the plasma in the direction of sample flow may also be controlled.
Additionally, it is desired to provide such a source is relatively inexpensive as compared to existng plasma loop sources.
The present invention provides an apparatus and a method for using the apparatus, in which at least three electrodes are distributed in a set circumferentially about a path along which a stream of sample material flows. Plasma gas is directed in the region of these electrodes and voltages are applied, relative to the tip distribution, to maintain a plasma generally surrounding the path.
In the preferred embodiment of the apparatus of the present invention, the electrodes are disposed in a horizontal plane normal to the travel path of such a sample stream. The electrode tips are disposed equidistant from each other about the sample stream and three-phase voltages are applied to them. Argon gas is projected cylindrically in the direction of stream flow outside the periphery of the electrode tips. The sample is entrained in argon gas to form an aerosol sample stream. Finally, argon gas is directed longitudinally along each electrode in order to cool it. This latter flow is limited to a flow rate which is insufficient to cause the plasma to block the centrally-disposed stream path. Optionally, a plurality of electrode sets may be disposed longitudinally along the sample stream travel path in order to extend the length of plasma produced thereby.
The positions of the electrodes may be altered and the applied voltages varied correspondingly to alter, the size and shape of the plasma produced. Also, it is anticipated, different numbers of electrodes may be distributed within each set and voltages applied appropriately to support plasma loops having other sizes and shapes.
These and additional objects and advantages of the present invention will be more clearly understood from a consideration of the drawings and the following detailed description of the preferred embodiment.